In the telecommunications cable industry, it has been found that system performance is improved by air pressurizing bundles of telephone wires housed within a sealed sheath tube, typically in the form of a tough plastic material. Unfortunately, the sheath can be perforated, allowing the pressurized air to rush out through the perforation and permitting ingress of water, dirt and the like. The out-rushing air generates a composite signal having sonic or ultrasonic frequency components, and sometimes both. To locate these perforations or leaks, remote sensing devices and associated electronics capable of responding to the leak generated noise have been developed.
Present techniques involve inserting an acoustically sensitive device along the sheath tube path and measuring the length of rod required to position the device adjacent to the leak. The device is typically sensitive to either (1) the sonic components or (2) some pre-selected narrow band of the ultrasonic component (e.g., 36-44 KHz) of noise vibrations generated by the air escaping through the perforation. According to these techniques, the vibration sensitive device, usually an acoustic sensor, is mounted on a flexible feeding apparatus and introduced into and along a length of the sheath tube. The sensor is electrically coupled to a metering device having a visual display which responds to output signals produced by the sensor when detection of a noise-emitting perforation is made. The distance of insertion of the sensor determines the location of the perforation.
It has been found, and believed heretofore unknown, that sheath tube perforations are capable of generating noise-emitting frequencies throughout the entire range of 0-100 KHz. Large perforations tend to generate noise vibrations having primarily a sonic frequency content; smaller perforations generate ultrasonic frequencies up to about 100 KHz. Thus, a disadvantage of either of the above approaches is that they fail to cover the full range of frequencies capable of being generated by noise-emitting perforations, i.e., 0-100 KHz.